Speech Intelligibility

The Speech Transmission Index (STI)

The Speech Transmission Index, or STI, gives a numerical value to speech transmission quality. As a purely objective measure, the STI can rate anywhere between 0 and 1. An index of 1 means that a speech has absolutely perfect intelligibility (which is virtually impossible, of course). The lower an STI value gets, the more degraded the quality of speech intelligibility becomes. While an STI rating of 1 is highly unlikely, there are standards that describe the qualities of certain ranges of scores. There are also subjective intelligibility tests (such as the PB word score) that relate back to the Speech Transmission Index. Some of these relations are shown in the table to the right.

Origins of the STI

Tammo Houtgast and Herman Steeneken invented the Speech Transmission Index in 1971 in the Netherlands. Since then, is has continued to evolve, as it still is today. It is particularly useful as it has a wide range of application areas. The STI is particularly useful for room acoustics of larger environments. This can include churches, auditoriums, or large conference rooms, although it functions incredibly well in smaller environments as well. In addition, the STI can be used to indicate speech intelligibility over telecommunication channels like radio transmissions and telephone lines.

Measuring the STI with Bedrock Devices

Although the Speech Transmission Index has been around for quite some time, measuring it has not always been practical. In fact, taking a full STI measurement has always taken about 15 minutes – until now. Bedrock is both happy and proud to announce that we have finally discovered a method of taking Full STI readings using the direct method in about 65 seconds. Although there have been other accurate methods of measuring the Speech Transmission Index quickly (namely STIPA), a Full STI measurement is the absolute most comprehensive measurement there is. Calling for 14 modulations per octave band, it is sampled far more than STIPA (which only calls for 2) in the modulation domain. These readings take just over a minute and can be done on any Bedrock SM50 or SM90 device.

Speech Intelligibility Rating Chart for Speech transmission index (STI) and CIS